In wireless network, stations share the wireless medium. This leads to competition for the wireless medium, as simultaneous messages can collide. To enable fair sharing, the IEEE802.11 standard has introduced the DCF and EDCA mechanisms in which stations execute the well known CSMA or CSMA/CA algorithms to access the medium. These mechanisms avoid collisions, to some extent, and enable relatively efficient usage of the medium. However, in V. Vishnevsky and A. I. Lyakhov, Cluster computing 5, 133-144, 2002, it is shown that these mechanisms lead to unfairness due to a seizing effect. With CSMA or CSMA/CA, before sending a message, a station tests the medium to know if it is available and to book it for sending the message afterwards. This operation is practiced by a station inside a time window called the contention window. A station that has just finished its transmission has an advantage to win the competition for the next transmission. Indeed, after a successful transmission, the CSMA or CSMA/CA algorithm prescribes that the contention window size is reset to the minimum window size. Thus, a station that has just been successful accesses the medium with a small window size and has an advantage over other stations which have not been successful recently in winning the contention. This can in turn lead to the unwanted situation that a backlogged station can monopolise the channel as it may gain exclusive access to the channel for a prolonged period of time.
This situation that occurs in standard WLANs, is experienced to an even higher degree in mesh networks. This is due to the increased density of wireless stations involved in such a mesh. Moreover, the consequences of this effect are even more serious in mesh networks than in standard WLANs and can lead to a dramatic throughput degradation as shown in S. Xu and T. Sadaawi, “Does the IEEE 802.11 MAC protocol work well in multihop wireless ad hoc networks?”, IEEE Communications Magazine June 2001, p. 130-137.
For instance, let assume that a backlogged station seizes the channel, because of the seizing effect. It can then send a lot of messages to its neighbouring station downstream. However, the neighbouring station cannot access the medium to a sufficient extent, because its upstream neighbour, the backlogged station, has seized the channel. Ultimately, the neighbouring station has no other option than to drop the incoming packets as its queues start overflowing. This situation leads to performance degradation.
In the drafted standard IEEE P802.11s/D1.00, November 2006 “Draft Amendment to Standard for Information Technology—Telecommunications and Information Exchange Between Systems—LAN/MAN Specific Requirements—Part 11: Wireless Medium Access Control (MAC) and physical layer (PHY) specifications: Amendment: ESS Mesh Networking.”, the problem of congestion management is anticipated by the creation, at the MAC level, of a broadcast “Neighbourhood Congestion Announcement” and/or a unicast “Congestion Control Request”. These messages use the Mesh Management frame format defined in paragraph 7.2.4.3 of the draft and are defined in the Mesh Management Action field (paragraph 7.4 and 7.3 of the draft).
However the draft does not specify the “congestion level” field nor the way this message must be used to manage congestion in the mesh network. In paragraph 11A.7 the draft describes some possible rules that can be used by a station to detect congestion: to monitor the transmission and receiving rate and the difference between these two aggregated rates, or to monitor the queue size, or a mix of both.
Upon receiving either a “Neighbourhood Congestion Announcement” or a “Congestion Control Request” message, the receiving node needs to reduce its effective MAC transmission rate accordingly by locally limiting its traffic. The local rate control mechanism may be based on dynamically adjusting EDCA parameters such as AIFSN, CWmin, or both.